When the holes for the dowels are centered and drilled at 90 degrees on a board, that makes for the strongest dowel joint. If you are joining 2 boards of the same thickness with this jig. The holes will be centered on both boards. So when you join them the boards will be flush. The plans are in Imperial and Metric.
To download the plans left click on the link below. Then click on the image of the plans. Then click on the download link at the top left of the page.
Self–Centering Doweling Jig Plans